The invention relates to an illuminable liquid crystal display, in particular in a motor vehicle, having a front polarization filter, a rear polarization filter and a liquid crystal cell, enclosed by means of two substrates respectively connected to electrodes, and a reflective element arranged behind the rear polarization filter.
Such a liquid crystal display is used in practice in order, on the one hand, to achieve a display in daytime operation by means of the incident ambient light (reflective operation), and on the other hand to enable an active illumination in nighttime operation by means of a light source. For this purpose, as transflector the reflective element has a settable transmission, with the result that the incident ambient light is reflected but, at the same time, the light emanating from the light sourcexe2x80x94except for an unavoidable attenuation dependent on the transmission of the transflectorxe2x80x94can be irradiated unimpeded from the rear side of the transflector. The possibility thus created of setting a desired brightness as a function of the ambient brightness by means of the light source also always ensures a brightness sufficient for reading the liquid crystal display in conjunction even with a variable ambient brightness.
However, it has proved to be disadvantageous in the prior art described that a variation in the reflective and/or transmissive properties of the reflective element is simultaneously associated with a variation in the color spectrum too. This can give rise to color distortions which impair the readability of the liquid crystal display.
The invention is therefore based on the problem of creating a liquid crystal display of the type mentioned at the beginning which minimizes color distortions as a consequence of switching from reflective to transflective or transmissive operation of the liquid crystal display. In particular, the readability of the liquid crystal display is also to be improved in this case.
This problem is solved according to the invention by virtue of the fact that the liquid crystal display has a light deflecting element for redirecting in the direction of the liquid crystal cell light laterally incident in a light coupling surface of the liquid crystal display. As a result, in the case of active illumination the reflective element will not require a transflective property, since irradiated light from a light source need not pass through the reflective element, and color distortions are thereby minimized. In this case, for the purpose of active illumination the light is fed, for example by means of an optical conductor, to the light coupling surface and redirected on the light deflecting element such that just like the ambient light in daytime operation it is reflected at the reflective element, designed as a mirror, for example, and radiated into the liquid crystal cell. In this case, there is also at the same time a substantial reduction in the brightness losses associated with the use of a transflector. Furthermore, it is possible in this case to achieve a compact, in particular very flat design in which, in addition, it is also possible to dispense with an expensive control unit for the reflective element. Owing to the compactness and low weight of the liquid crystal display according to the invention, the latter is suitable, in particular, for use in motor vehicles, since the aims of effective use of space and maximum weight reduction are regarded as imperative, particularly with modern vehicles.
The light deflecting element and the light coupling surface can be arranged on a separate component which is added depending on the individual requirements placed on the liquid crystal display. Another particularly favorable embodiment of the invention results, by contrast, when the light deflecting element is arranged on a surface, facing the liquid crystal cell, of at least one of the two substrates. This eliminates an additional component, with the result that the overall space required can be further reduced. In this case, the substrate serves simultaneously as optical conductor for the optical radiation coupled in by means of the light coupling surface, and thus permits a smooth light distribution with uniform brightness.
A particularly advantageous development of the invention results in this case by virtue of the fact that the light deflecting element is arranged on a side of the rear polarization filter averted from a viewer of the liquid crystal display. Consequently, the light is deflected directly in the direction of the front side of the liquid crystal display, so that no redirection of the light on the reflective elements is required in actively illuminated operation. The individual components of the liquid crystal display are passed by the coupled-in light only respectively in one direction in this case, thus permitting further reduction in possible brightness losses.
A particularly advantageous refinement of the invention results when one of the substrates has the light coupling surface, and when the light coupling surface is arranged on an end face of the substrate having the light coupling surface. In this case, the corresponding substrate simultaneously permits as an integral component on the one hand the coupling of additional optical radiation by means of the light coupling surface, and on the other hand the unhindered passage of the ambient brightness entering through the front side of the liquid crystal display. A substrate designed in such a way can therefore be exchanged without difficulty for a conventional substrate and therefore does not require any expensive new design. Further embodiments in which both substrates are fitted with corresponding light coupling surfaces and light deflecting elements permit illumination of optionally different brightness, for example.
Another particularly favorable development of the invention results when an end face of the liquid crystal display opposite the light coupling surface is provided with a reflector. Consequently, brightness losses owing to the uncontrolled escape of the coupled-in light at a side averted from the light coupling surface can be prevented. Such a mirror surface, designed as a coating, for example, therefore permits a further improvement in efficiency in the case of active illumination. Further lateral surfaces and also the edge surfaces projecting beyond a display surface of the liquid crystal display can additionally also be fitted in this case with such a reflector.
A particular simple embodiment of the invention in this case is one in which the light deflecting element is designed as a structured surface. A light deflecting element designed in such a way essentially permits unchanged external dimensions of the liquid crystal display. In this case, a structured surface designed, for example, as a microstructure simultaneously prevents local differences in brightness and therefore leads to a readability that is further improved. The structured surface can also be designed for this purpose as a texture or lens structure, for example, and be arranged both on the surface and in the interior of a transparent element.
By means of an optical conductor, it is possible to radiate into the light coupling surface comparatively bright ambient light which is not directly accessible to the liquid crystal display, or the light of independent illuminating units. A particularly advantageous development of the invention is, by contrast, also provided when the liquid crystal display has a light source radiating into the light coupling surface. Such a light source permits the required intensity of illumination to be adapted optimally in accordance with the respectively prevailing operating conditions. For this purpose, the light source can, for example, also be electronically dimmable, in order thus to permit a smooth transition between passive illumination by means of ambient brightness and active illumination by means of the light source.
A particularly well suited development of the invention results in this case by virtue of the fact that the light source is a discharge lamp. Because of its strong brightness in simultaneous conjunction with comparatively low thermal emission, the discharge lamp permits a readability even in the case of negative display of the liquid crystal display in which the driven segments appear in bright relief against the otherwise dark background.
By contrast, another advantageous development of the invention is achieved when the light source has a light-emitting diode (LED). Owing to the low power consumption and the longevity of a liquid crystal display of such design, it is also possible to use it away from the permanent line supply. Use is possible in this case, for example, with portable units and units operated by battery or rechargeable battery. The light-emitting diode (LED) is suitable in this case chiefly for positive display, in which the driven segments are displayed as darkened surfaces on the otherwise brightly illuminated background.
In accordance with another advantageous development of the invention, the color distortions of the liquid crystal display as a consequence of the switching from reflective to transflective or transmissive operation can be virtually completely excluded in wide limits when the color spectrum of the light source is adapted to the color spectrum of the incident light falling into the liquid crystal display. Since the color spectrum of the incident light, which usually concerns ambient brightness (that is to say sunlight), can vary as a function of season and time of day, for example, it is not possible under all conditions to ensure with the aid of a light source having a fixed color spectrum that there is an identical correspondence between its color spectrum and that of the incident light. It would be possible to provide a remedy here by providing that the color spectrum of the light source can be set, but this is very complicated and cost intensive. However, it is possible to assume an average color spectrum, constant within wide limits, of the incident light, with the result that the color distortions can be excluded for virtually all relevant applications in the event of adaptation of the light source thereto.
In accordance with another development of the invention, it is also particularly advantageous when a spectral filter is arranged upstream of the front polarization filter and/or between the light source and the light coupling surface. The color spectrum of the incident light and/or of the light source is thereby established in a simple way, and color distortions of the display are thereby reliably excluded.
The invention permits various embodiments.